The concept of using various adsorbents, including various natural and synthetic zeolite molecular sieve materials, in processes for effecting physical separations of various mixtures has been known and used both experimentally and commercially for quite some time. For example, S. A. Coviser, (The Oil and Gas Journal, Dec. 6, 1965, pp. 130-32) discussed the adsorption capabilities of silica gel, copper-impregnated activated carbon, type 5A molecular sieves and type 13X molecular sieves with respect to the removal of mercaptan sulfur from natural gas in the vapor phase.
In 1967, L. F. Fominykh, et al., (Khimiya i Tekhnologiya Topliv i Masel, No. 4, pp. 8-10, April 1967) discussed the use of X-type zeolites for the adsorptive separation of benzene from an artificially prepared binary mixture of benzene and n-heptane containing about 12.2% by weight benzene. The separation, which was performed either in vapor phase or liquid phase under dynamic conditions, was said to have reduced the level of benzene in the binary mixture down to about 0.24% by weight.
Another disclosure which relates to the separation of a single aromatic material from a single paraffinic material is contained in Milton, U.S. Pat. No. 3,078,643. In accordance with this Milton patent, toluene can be separated from a vapor mixture of, for example, toluene and n-hexane by contacting the vapor mixture with a bed of zeolite X-type adsorbent material, the pores of which are sufficiently large to adsorb toluene and n-hexane, and thereafter discharging a toluene-depleted vapor stream from the zeolite bed. As indicated in this patent, the level of toluene in the vapor mixture can be reduced to a level of about 3% by weight.
In connection with processes of the type disclosed in the above Fominykh, et al., article and Milton patent, it is noted that the separation of binary systems of n-paraffin-aromatic mixtures has been investigated by researchers for many years. The primary objective of such research generally is either to provide a process of separation for a specific industrial application (as in the case of Milton) or to provide binary data for various systems in an attempt to arrive at a model for the possible prediction of anticipated results for multicomponent adsorption processes. As will be seen from the discussion hereinbelow, the multicomponent separations which are accomplished by the present invention are much more complicated and general in nature than the simple and specific binary mixture separations disclosed, for example, in Milton and Fominykh, et al.
In addition to dealing with simple binary systems, there are a number of prior disclosures relevant to multicomponent separations of aromatics or nonaromatics from saturated hydrocarbons and/or olefins. In many cases, these prior disclosures relate to separation processes which are similar in some respects to the present process, but which, in other important respects, are greatly different therefrom. For example, Epperly, et al., U.S. Pat. No. 3,228,995 relates to a process for purifying C.sub.10 to C.sub.25 hydrocarbons containing at least one impurity selected from aromatics, sulfur, and color bodies, wherein the impure hydrocarbons are contacted with a type X zeolite. However, unlike the present process, the process described in this Epperly, et al. patent requires that at least a portion of the absorbed impurities be desorbed with a gaseous displacing agent, such as gaseous SO.sub.2, NH.sub.3, CO.sub.2, C.sub.1 -C.sub.5 alcohols, methyl chloride, or the like or, preferably, a gaseous amine having the formula ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 are hydrogen or a C.sub.1 -C.sub.5 alkyl radical; that the desorbed portion be recycled over the zeolite bed; that the remaining portion of the adsorbed components be desorbed with a gaseous displacing agent; and that the desorbing and recycling be continued for as many as 450 cycles or more until the desired degree of impurity removal has been attained. Moreover, the process described in this Epperly, et al. patent preferably is carried out in the vapor phase and at temperatures on the order of from about 400.degree. to about 800.degree. F.
Another Epperly, et al. patent, i.e., U.S. Pat. No. 3,063,934, relates to the removal of aromatics, olefins and sulfur from a naphtha feed which is to be used for isomerization and paraffin alkylation. In accordance with this patent, a C.sub.5 /C.sub.6 naphtha feed is contacted with a type X molecular sieve at a temperature of from about 70.degree. to 500.degree. F., and preferably from about 200.degree. to 350.degree. F., to adsorb aromatics, olefins and sulfur therefrom. The aromatics are desorbed from the molecular sieve material during a heat-purge phase wherein the sieve material is contacted with isomerate vapors from an isomerization reactor, which vapors have been heated to about 650.degree. F.
Still other disclosures which relate to the use of molecular sieve materials in separation processes and which are of background interest with respect to the present invention include Milton, U.S. Pat. No. 2,882,244; Tuttle, et al., U.S. Pat. No. 2,978,407; Fleck, et al., U.S. Pat. No. 3,182,017; Ludlow, et al., U.S. Pat. No. 3,205,166; Peck, et al., U.S. Pat. No. 3,265,750; Epperly, et al., U.S. Pat. No. 3,468,791; Shively, et al., U.S. Pat. No. 3,658,696; Epperly, et al., U.S. Pat. No. 3,558,732; Neuzil, U.S. Pat. No. 3,558,730; Eberly, Jr., et al., U.S. Pat. No. 3,485,748; Francis, U.S. Pat. No. 3,726,792; French Pat. No. 1,382,149 (isolation of aromatic hydrocarbons from naphtha and kerosene cuts by using type X molecular sieves); E. L. Clark, (Oil and Gas Journal, No. 46, pp. 178-84, Nov. 12, 1962); A. Z. Dorogochinskii, (Khimya i Tekhnologiya Topliv i Masel, No. 8, pp. 4-6, August 1973); L. C. Waterman, (Chem. Eng. Progr., Vol. 61, No. 10, pp. 51-57, Oct. 1965); and A. G. Martynenko, Khimya i Tekhnologiya Topliv i Masel, No. 8, pp. 11-12, August 1969).